1. Field of the Invention
The present invention relates to a vibration control apparatus for an automotive vehicle and particularly to a vehicle control apparatus for suppressing vibrations such as a front/rear vibration, left/right vibration and up/down vibration, caused by a driver's operation of an accelerator, steering and brake.
2. Description of the Related Art
One of the conventional vibration control is made through a suspension, wherein a squat and dive during driving and braking are suppressed by a suspension geometry, or wherein the vibrations are suppressed by changing a damper spring or stiffness characteristics of the suspension, as disclosed in JP2001-30728A, pages 2-8, 11 and 12, FIG. 6.
Further, a driving force control method for suppressing the vibration of the vehicle body due to the driving force caused by the accelerator operation is disclosed in JP9-112329A, 1997, pages 2-4 and 6, FIG. 3.
In the above-mentioned driving force control method, a higher frequency spectrum of the accelerator operation signal is damped by applying, to the accelerator operation signal, a low pass filter for a first order delay or high pass filter for a first order advance, whereby any shock due to an abrupt change in the driving force when the driver abruptly operates the accelerator is suppressed. Further, the front/rear vibration uncomfortable for a passenger is suppressed by damping natural vibration components in a wheel driving system.
Further, it is disclosed, in JP3-47444A, 1991, pages 1-3, FIG. 4 and JP59-23037A, 1984, pages 1-3, FIG. 1, that the engine output is compensated in order to reduce the vehicle body vibration by detecting the vehicle body vibration by using an acceleration sensor, in such a manner that the engine output is compensated by a vibration of which phase is reverse to that of the vehicle vibration, if the vehicle vibration becomes greater than a prescribed vibration.
Recently, platforms (vehicle body type) of the engine and body have been unified for a plurality of kinds of vehicles. Accordingly, it is advantageous that a unified vibration control model for the plurality of kinds of vehicles is used, because the vehicle control is achieved by merely changing control parameters and fixed parameters.
As for the vehicle performances, a greater change in the ground loads of the wheels due to the vehicle vibration disadvantageously affects fundamental performances such as running, turning and stopping of the vehicle. For example, if the ground load of one of the wheels is decreased, a reaction force from the road surface is decreased, thereby possibly decreasing the braking force or turning force. Accordingly, it is required to suppress to the utmost the vehicle vibration which is one of the factors which changes the ground load.
In light of the above-mentioned vehicle vibration suppression, JP2001-30728A suppresses the vibration by using a model of vibration on the springs of the front and rear wheels corresponding the suspension system of the front and rear wheels, wherein only the damping force of the shock absorber of the suspension is compensated, even when the vibration is caused by the driver's operation of the steering, accelerator or brake.
Further, the above-mentioned model of vibration on the springs of the front and rear wheels is a dynamic model merely for deciding how the position of the center of gravity of the vehicle is changed by the suspension operation. In general, the vibrations uncomfortable to the passenger are threefold, i.e., a first (several Hertz (1 to 2 Hertz)) on-spring vibration of the vehicle body itself, second (about 10 Hertz) under-spring vibration and third (35 to 40 Hertz) vibration (tire vibration) of the wheels due to the reaction force from the road surface along the vertical direction and/or the twisting direction. The first vibration is most uncomfortable, the second vibration is second most uncomfortable and the third vibration is least uncomfortable than the first and second vibration, although the factor which causes the vibration is based on the reaction force received by the tire from the road surface and the reaction force is based on the driving/braking/turning forces caused by engine output/braking operation/steering operation, respectively. The reaction force causes the under-spring vibration under the suspension which causes the on-spring vibration. Accordingly, the vibration model might preferably have priority over the third vibration in accordance with an order of generation and transfer of the vibration, while the vibration control might preferably have priority over the first vibration in accordance with the uncomfortable degrees of the vibration.
In light of the above-mentioned model and control of the vibration, JP2001-30728A merely employs the two wheel on-spring vibration model which analyzes dynamically the suspension portion. Therefore, it can not achieve an optimum control of the vibration uncomfortable to the passenger.
Further, JP9-112329A, 1997, JP3-47444A, 1991 and JP59-23037A, 1984 do not include any model for suppressing the vibration. For example, in JP9-112329A, 1997, the accelerator operation signal is directly filtered in order to suppress the vehicle vibration. In this case, if the filter characteristics are not adaptively changed, the vehicle vibration may possibly be gradually diverged greatly, e.g., when the accelerator pedal is repeatedly stepped down strongly and weakly. Further, as for the adaptive change of the filter characteristics, it is not easy to determine, for example, when the filtering range should be recovered after narrowing it.